In an aqueous solution, precipitation is the process of transforming a dissolved
substance into an insoluble
solid
Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structur ...
from a
super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemical reagent causing the solid to form is called the ''precipitant''.
The clear liquid remaining above the precipitated or the centrifuged solid phase is also called the 'supernate' or 'supernatant'.
The notion of precipitation can also be extended to other domains of chemistry (organic chemistry and biochemistry) and even be applied to the solid phases (''e.g.'', metallurgy and alloys) when solid impurities
segregate from a solid phase.
Supersaturation
The precipitation of a compound may occur when its concentration exceeds its
solubility
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubi ...
. This can be due to temperature changes, solvent evaporation, or by mixing solvents. Precipitation occurs more rapidly from a strongly
supersaturated
In physical chemistry, supersaturation occurs with a solution when the concentration of a solute exceeds the concentration specified by the value of solubility at equilibrium. Most commonly the term is applied to a solution of a solid in a li ...
solution.
The formation of a precipitate can be caused by a chemical reaction. When a
barium chloride solution reacts with
sulphuric acid, a white precipitate of
barium sulfate is formed. When a
potassium iodide solution reacts with a
lead(II) nitrate solution, a yellow precipitate of
lead(II) iodide is formed.
Nucleation
An important stage of the precipitation process is the onset of
nucleation. The creation of a solid
particle
In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.
They vary greatly in size or quantity, from ...
implies the formation of an
interface with the solution. This involves
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...
changes depending on the dissolution reaction
free energy (
endothermic
In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. ...
or
exothermic
In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity ...
process accompanied by an
entropy
Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...
increase) and the relative
surface energy
In surface science, surface free energy (also interfacial free energy or surface energy) quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less ener ...
developed between the solid and the solution. If energy changes are not favorable, or without suitable nucleation sites, no precipitation occurs and the solution remain supersaturated.
Inorganic chemistry
Precipitation in aqueous solution
A common example of precipitation reaction in aqueous solution is that of
silver chloride
Silver chloride is a chemical compound with the chemical formula Ag Cl. This white crystalline solid is well known for its low solubility in water (this behavior being reminiscent of the chlorides of Tl+ and Pb2+). Upon illumination or heat ...
. When
silver nitrate
Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...
(AgNO
3) is added to a solution of
potassium chloride (KCl) the precipitation of a white solid (AgCl) is observed.
:
AgNO3 + KCl -> AgCl (v) + KNO3
The
ionic equation allows to write this reaction by detailing the
dissociated
Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an acid ...
ions present in aqueous solution.
:
Ag+ + NO3^- + K+ + Cl^- -> AgCl (v) + K+ + NO3^-
Reductive precipitation
The
Walden reductor is an illustration of a
reduction reaction directly accompanied by the precipitation of a less soluble compound because of its lower chemical valence:
:
Cu + 2 Ag+ -> Cu^2+ + 2 Ag
The Walden reductor made of tiny
silver
Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...
crystals obtained by the immersion of a
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
wire into a solution of
silver nitrate
Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...
is used to reduce to their lower valence any metallic ion located above the silver couple in the
redox potential scale.
Precipitate colors
Many compounds containing
metal
A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
ions produce precipitates with distinctive colors. The following are some typical colors for various metals. However, many of these compounds can produce colors very different from those listed.
Many compounds often form white precipitates.
Anion/cation qualitative analysis
Precipitate formation is useful in the detection of the type of
cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
in a
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...
. To do this, an
alkali first reacts with the unknown salt to produce a precipitate that is the
hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. ...
of the unknown salt. To identify the cation, the color of the precipitate and its solubility in excess are noted. Similar processes are often used in sequence – for example, a
barium nitrate solution will react with
sulfate ions to form a solid
barium sulfate precipitate, indicating that it is likely that sulfate ions are present.
Colloidal suspensions
Without sufficient attraction forces (''e.g.'',
Van der Waals force
In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and ...
) to aggregate the solid particles together and to remove them from solution by gravity (
settling
Settling is the process by which particulates move towards the bottom of a liquid and form a sediment. Particles that experience a force, either due to gravity or due to centrifugal motion will tend to move in a uniform manner in the direction ...
), they remain in
suspension and form
colloids.
Sedimentation can be accelerated by high speed
centrifugation. The compact mass obtained so is sometimes referred to as a 'pellet'.
Digestion and precipitates ageing
Digestion, or ''precipitate ageing'', happens when a freshly formed precipitate is left, usually at a higher
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
, in the solution from which it precipitates. It results in purer and larger recrystallized particles. The physico-chemical process underlying digestion is called
Ostwald ripening.
Organic chemistry
While precipitation reactions can be used for making
pigment
A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...
s, removing ions from solution in
water treatment
Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, inc ...
, and in
classical qualitative inorganic analysis, precipitation is also commonly used to isolate the products of an organic reaction during
workup and purification operations. Ideally, the product of the reaction is insoluble in the solvent used for the reaction. Thus, it precipitates as it is formed, preferably
forming pure crystals. An example of this would be the synthesis of
porphyrins in refluxing
propionic acid. By cooling the reaction mixture to room temperature, crystals of the porphyrin precipitate, and are collected by filtration on a Büchner filter as illustrated by the photograph here beside:
Precipitation may also occur when an ''antisolvent'' (a solvent in which the product is insoluble) is added, drastically reducing the solubility of the desired product. Thereafter, the precipitate may be easily separated by
decanting
Decantation is a process for the separation of mixtures of immiscible liquids or of a liquid and a solid mixture such as a suspension. The layer closer to the top of the container—the less dense of the two liquids, or the liquid from which th ...
,
filtration, or by
centrifugation. An example would be the synthesis of Cr
3+ tetraphenylporphyrin chloride: water is added to the
dimethylformamide (DMF) solution in which the reaction occurred, and the product precipitates. Precipitation is useful in purifying many other products: ''e.g.'', crude
bmim
1-Butyl-3-methylimidazolium hexafluorophosphate, also known as BMIM-PF6, is a viscous, colourless, hydrophobic and non-water-soluble ionic liquid with a melting point of -8 °C. Together with 1-butyl-3-methylimidazolium tetrafluoroborate, BMI ...
-Cl is taken up in
acetonitrile
Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound with the formula and structure . This colourless liquid is the simplest organic nitrile ( hydrogen cyanide is a simpler nitrile, but the cyanide anion is not clas ...
, and dropped into
ethyl acetate, where it precipitates.
Biochemistry
Protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s purification and separation can be performed by precipitation in changing the nature of the solvent or the value of its
dielectric constant
The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insula ...
(''e.g.'', by replacing water by
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...
), or by increasing the
ionic strength of the solution. As proteins have complex tertiary and quaternary structures due to their specific folding and various weak intermolecular interactions (''e.g.'', hydrogen bridges), these superstructures can be modified and proteins denaturated and precipitated. Another important application of an antisolvent is in
ethanol precipitation Ethanol precipitation is a method used to purify and/or concentrate RNA, DNA, and polysaccharides such as pectin and xyloglucan from aqueous solutions by adding ethanol as an antisolvent.
DNA precipitation Theory
DNA is polar due to its h ...
of
DNA.
Metallurgy and alloys
In solid phases, precipitation occurs if the concentration of one solid is above the solubility limit in the host solid, due to e.g. rapid quenching or
ion implantation, and the temperature is high enough that diffusion can lead to
segregation into precipitates. Precipitation in solids is routinely used to synthesize
nanoclusters.
In
metallurgy, precipitation from a
solid solution is also a way to
strengthen alloys.
Precipitation of
ceramic phases in
metallic alloys such as
zirconium hydrides in
zircaloy
Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance. One of the ...
cladding of
nuclear fuel
Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.
Most nuclear fuels contain heavy fissile actinide elements that are capable of undergo ...
pins can also render metallic alloys brittle and lead to their mechanical failure. Correctly mastering the precise temperature and pressure conditions when cooling down
spent nuclear fuels is therefore essential to avoid damaging their cladding and to preserve the integrity of the spent fuel elements on the long term in dry storage casks and in geological disposal conditions.
Industrial processes
Hydroxide precipitation is probably the most widely used industrial precipitation process in which
metal hydroxides are formed by adding
calcium hydroxide (''slaked lime'') or
sodium hydroxide (''caustic'' ''soda'') as precipitant.
History
Powders derived from different precipitation processes have also
historically
History (derived ) is the systematic study and the documentation of the human activity. The time period of event before the invention of writing systems is considered prehistory. "History" is an umbrella term comprising past events as well ...
been known as 'flowers'.
See also
*
Coprecipitation
*
Effervescence
Effervescence is the escape of gas from an aqueous solution and the foaming or fizzing that results from that release. The word effervescence is derived from the Latin verb ''fervere'' (to boil), preceded by the adverb ''ex''. It has the same li ...
, the "up-arrow"
*
Salting in
*
Salting out
Salting out (also known as salt-induced precipitation, salt fractionation, anti-solvent crystallization, precipitation crystallization, or drowning out) is a purification technique that utilizes the reduced solubility of certain molecules in a s ...
References
Additional reading
*
External links
Precipitation reactions of certain cationsDigestion InstrumentsA Thesis on pattern formation in precipitation reactions
{{DEFAULTSORT:Precipitation (Chemistry)
Liquid-solid separation